Digital data communication systems employing a carrier that is modulated by data at a transmitter and demodulated at a receiver to decode the transmitted data are well known. Several different types of modulation and demodulation are known and used. This invention pertains to a type of modulation-demodulation which employs data transmission through multiple differential phase-shift-keying ("MDPSK") at the modulator and a comparable differential detection at the system's receiver or demodulator.
Modems, a contraction for modulators and demodulators, are commonly used at both ends of a data communication transmission system. When one modem physically moves relative to another modem in a transmission link, the transmitted signals experience a Doppler effect. Typically Doppler effects are experienced in an earth to space satellite link and cause an effective change of frequency of a received signal due to the relative velocity of the link's transmitter with respect to the link's receiver. The Doppler effect must be rectified in order to achieve efficient data transmission and reception.
A prior art search related to the invention discloses some typical approaches to Doppler correction. Not one of the references located in the search, however, discloses or suggests an open-loop Doppler frequency estimation derived from an incoming signal. Moreover, open-loop correction for a Doppler effect in a MDPSK system having an improved filtering and differential detection configuration is not disclosed in the prior art.
The following search-located U.S. Pat. Nos. include: Heard et al 4,520,493, Poklemba 4,419,759, Poklemba et al 4,472,817 and Mehrgardt et al 4,663,595.
Heard '493 discloses a complex closed-loop Doppler correction scheme that decodes and then attempts to correct for the next expected Doppler effect. Such closed loop tracking is not feasible in certain Doppler-related-applications e.g., a multipath fading environment. Poklemba et al '817 discloses open loops but does not disclose Doppler correction in a differential detection system. Poklemba '759 is a typical phase-locked loop system. Mehrgardt '595 is of marginal interest only in that it discloses delays and summers in a form typically known in the modulator/demodulator art.
A multipath fading . environment creates uncertainties in the carrier phase and/or frequency. These uncertainties essentially eliminate the known techniques which use a phase and/or frequency tracking synchronization loop. Moreover, the above-noted uncertainties may be further masked by phase jitter on the carrier reference. This additional transmission impairment adds further complexity to the system operation. Differential detection employs, as a demodulation reference, the phase and frequency of the carrier corresponding to the previous transmitted data symbol. The deleterious effects in a multipath fading environment are largely avoided by that differential system approach because there is no requirement that the carrier must be synchronized during each present symbol interval.
Another known transmission impairment is intersymbol interference (ISI). It is known to use pulse shaping so that the ISI at a receiver is at a minimum at the sampling point for data detection by the receiver Splitting signal filtering between the transmitter and receiver locations is a known way of achieving pulse shaping and minimum ISI.
For MDPSK systems it is common to strive for what is known as a 100% raised-cosine spectrum at the decoder's input. In an MDPSK system such shaping will achieve a satisfactory signal to noise ratio for center sampling at the decoder or demodulator. Various types of multilevel coding and decoding schemes have been introduced in efforts to achieve an optimum signal to noise ratio for such center sampling. Included within these multilevel schemes is the so-called duobinary signaling, a partial response signaling scheme as taught in A. Lender, "The Duobinary Technique for High Speed Data Transmission," IEEE Transactions on Communication Electronics, Vol. 82, May 1963, pp. 214-218.